[libvirt] [PATCH v5] Add functions for handling exponential backoff loops.

[Richard W.M. Jones]

On Fri, Apr 15, 2016 at 04:59:03PM +0200, Peter Krempa wrote:
> On Fri, Apr 15, 2016 at 16:45:07 +0200, Michal Privoznik wrote:
> > On 15.04.2016 13:53, Richard W.M. Jones wrote:
> > > In a few places in libvirt we busy-wait for events, for example qemu
> > > creating a monitor socket.  This is problematic because:
> > > 
> > >  - We need to choose a sufficiently small polling period so that
> > >    libvirt doesn't add unnecessary delays.
> > > 
> > >  - We need to choose a sufficiently large polling period so that
> > >    the effect of busy-waiting doesn't affect the system.
> > > 
> > > The solution to this conflict is to use an exponential backoff.
> > > 
> > > This patch adds two functions to hide the details, and modifies a few
> > > places where we currently busy-wait.
> > > ---
> > >  src/fdstream.c           | 10 +++---
> > >  src/libvirt_private.syms |  2 ++
> > >  src/qemu/qemu_agent.c    |  9 +++---
> > >  src/qemu/qemu_monitor.c  | 10 +++---
> > >  src/util/virtime.c       | 81 ++++++++++++++++++++++++++++++++++++++++++++++++
> > >  src/util/virtime.h       | 11 +++++++
> > >  6 files changed, 111 insertions(+), 12 deletions(-)
> > 
> > ACK. Sorry for making you send v5 to such simple patch.
> 
> GCC 5.3.0 doesn't like this very much:
> 
> qemu/qemu_agent.c:238:8: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>      if (ret != 0) {
> 
> qemu/qemu_monitor.c:369:8: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>      if (ret != 0) {
>
> The static analyzer considers the possibility that the while loop will
> never execute, which would be possible in a very strange lockup of the
> host.

GCC 6 doesn't warn, which doesn't seem right to me, unless GCC is
doing some incredibly clever dataflow analysis.

> Please initialize ret to -1 before pushing in those two functions.

Done, thanks.

Rich.

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